1. Field of the Invention
The present invention generally relates to electron beam lithography tools and, more particularly, to electron beam projection lithography tools of high throughput for semiconductor integrated circuit manufacture.
2. Description of the Prior Art
It has been recognized that increasing integration density and device proximity can enhance both performance and manufacturing economy of integrated circuits. Accordingly, device dimensions therein have been scaled to smaller design rules and feature size regimes to the limit of available lithographic processes to define device dimensions and locations. At the present state of the art, lithographic resists must be exposed with electron beams to produce feature sizes and resolutions that allow improved performance and manufacturing economies to be realized to the greatest possible extent.
The manufacturing economies at increased integration density generally derive from the fact that smaller device dimensions allow increased numbers of devices to be formed on chips which are diced from a wafer subjected to a sequence of processing steps. Thus the process cost is distributed over a greater number of devices. However, a greater number of devices per chip implies an increase in the number of pixels to be exposed by a similar factor. At the present state of the art, the number of pixels which must be exposed on a single chip can exceed one billion or more while the number of pixels that can be simultaneously exposed by a single exposure of an electron beam projection system is limited to a few hundred thousand or few million. Therefore, several hundred to several thousand individual exposures must be made in rapid sequence for a single high density integrated circuit chip and the electron beam must be blanked between exposures.
Thus for acceptable levels of throughput for electron beam exposure tools, exposure time must be short and beam energy and exposure current must be high. Blanking by altering potential on a grid in the manner of cathode ray tubes is not feasible in view of the high voltage required to stop the beam relative to the available slew rate of amplifiers, the potential for causing variability of electron flux of the beam and the alteration of thermal conditions of the exposure tool when the beam is stopped. Accordingly, blanking is generally accomplished by deflecting the beam away from an aperture in one or more plates which intercept the beam. The power in the beam may be up to several hundred watts, most of which must be dissipated at aperture stops along the beam path, particularly when the beam is blanked and the plate (often referred to as an aperture diaphragm) surrounding the aperture prevents the beam from reaching the target (e.g. wafer).
It should also be appreciated that electron beam projection systems must be extremely stable dimensionally since the positional error tolerable in an exposure is a fraction of the minimum feature size to be produced so that the respective patterns in adjacent exposure areas can be adequately stitched together for electrical continuity. Accordingly, any positional variation in the concentrated, focused, high power beam is a potential source of instability due to the location(s) where the beam is at least partially incident and fraction of the beam power which must be dissipated at such location(s).
For blanking, substantially the entire beam power is dissipated at the location on the blanking aperture or aperture diaphragm to which the beam is deflected. The blanking duty cycle is dictated by the operational conditions of the system and thus cannot be altered to limit power dissipation. The location available on the blanking aperture to which the beam can be deflected is also limited in range to a region between a small multiple of the beam diameter (to assure complete beam stoppage) and a maximum deflection limited by deflector sensitivity and available deflector excitation power.
Standard blanking arrangements deflect the beam in a meridianal plane and limits the area of the aperture diaphragm which is heated by the beam to one side of the aperture therein; invariably leading to a lateral shift of the aperture and an alteration of beam position at the target by a distance which, in most cases, will be significant relative to or exceed the minimum feature size of the integrated circuit design and thus prevent correct stitching of sub-field patterns or cause blurring or uneven intensity of the image due to aperture motion with temperature change that is reflected in beam position. Depending on the blanking period, the aperture diaphragm can be destroyed or partially melted due to the extreme power density over a limited area.
(A hollow beam technique as disclosed in U.S. Pat. No. 5,834,783 where the hollow beam is developed by a blocking disk suspended in an aperture of the aperture diaphragm with delicate spokes would be even more severely affected. With such an annular aperture, uneven heating of the spokes severely distorts the beam while the blocking disk is less able to dissipate heat.)
More generally, it should be appreciated that the significance of any dimensional instability of the electron beam projection system is related to the minimum feature size which must be produced and the effects described in the preceding paragraphs, although potentially foreseeable, may be negligible or undetectable for some sub-micron feature size regimes close to the current state of the art. That is, when maximum integration density and highest throughput levels are not aggressively pursued, adequate levels of throughput can be achieved at, for example, quarter-micron feature sizes at beam current levels and blanking duty cycles which would ordinarily maintain any shift of beam position location to dimensions which are a small fraction of a quarter-micron, if detectable at all. However, the inventors have found that when minimum feature size and maximum throughput are aggressively pursued (e.g. a 0.1 micron minimum feature size and beam power levels of 5 KW/cm2 or more), the beam location shift may become quite significant, as will be more quantitatively discussed in connection with a description of the invention and, moreover, damage to elements of the e-beam exposure system may result.
Accordingly, it is seen that known blanking techniques do not adequately support acceptable levels of throughput at currently feasible and future feature sizes in integrated circuit manufacturing processes. No alternative has existed other than increased deflector excitation power and even that expedient would be of limited effectiveness to reduce the above-described effects on dimensional stability of the tool.
It is therefore an object of the present invention to provide a technique of blanking an electron beam in an electron beam lithography tool which does not cause distortion of the geometry and provides stability to the geometry thereof.
It is a further object of the invention to provide a method of operating an electron beam projection lithography tool and method of manufacturing high-density integrated circuits with improved positional accuracy and sub-field stitching at high throughput.
In order to accomplish these and other objects of the invention, a method of operating a charged particle beam device and manufacturing semiconductor integrated circuits is provided including steps of generating a beam of charged particles at an aperture in an aperture diaphragm, deflecting the beam of charged particles away from the aperture, deflecting the beam along a locus on a surface of the aperture diaphragm which is approximately symmetrical around the aperture, and deflecting the beam from a location on the locus to the aperture.
In accordance with another aspect of the invention, an apparatus for blanking an electron beam is provided including an aperture diaphragm having an aperture, a source providing a charged particle beam at the aperture, and an arrangement located above the aperture diaphragm for deflecting the charged particle beam in a two-dimensional pattern on the aperture diaphragm along a locus spaced approximately symmetrically around the aperture in the aperture diaphragm.